Nonvolatile semiconductor memory device and method for manufacturing the same

ABSTRACT

On a silicon substrate is formed a stacked body by alternately stacking a plurality of silicon oxide films and silicon films, a trench is formed in the stacked body, an alumina film, a silicon nitride film and a silicon oxide film are formed in this order on an inner surface of the trench, and a channel silicon crystalline film is formed on the silicon oxide film. Next, a silicon oxide layer is formed at an interface between the silicon oxide film and the channel silicon crystalline film by performing thermal treatment in an oxygen gas atmosphere.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 14/461,013 filed Aug. 15, 2014,which is a division of U.S. Ser. No. 13/044,673 filed Mar. 10, 2011 (nowU.S. Pat. No. 8,841,183 issued Sep. 23, 2014), which is a continuationof U.S. Ser. No. 12/575,906 filed Oct. 8, 2009 (now U.S. Pat. No.7,927,953 issued Apr. 19, 2011), and claims the benefit of priorityunder 35 U.S.C. §119 from Japanese Patent Application No. 2008-314187filed Dec. 10, 2008; the entire contents of each of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a nonvolatile semiconductor memory device inwhich a channel film made of silicon is formed on an insulating film,and a method for manufacturing the same.

2. Background Art

Recently, as an alternative to the planar nonvolatile semiconductormemory device, a nonvolatile semiconductor memory device in which aplurality of MONOS (metal-oxide-nitride-oxide-silicon) type memory cellsare three-dimensionally stacked has been developed (see, e.g.,JP-A-2008-171918(Kokai)). This nonvolatile semiconductor memory deviceis manufactured as follows. Gate electrodes and interlayer insulatingfilms are alternately formed on a substrate to form a stacked body, anda through hole extending in the stacking direction of the stacked bodyis formed therein. A charge block film, a charge storage film, and atunnel insulating film are formed in this order on the inner surface ofthe through hole. Subsequently, silicon is deposited by the CVD(chemical vapor deposition) process, for instance. Thus, a verticallyextending silicon pillar is buried in the through hole, and serves as achannel of memory cells.

However, in this manufacturing method, the silicon pillar is formed bydepositing silicon on the tunnel insulating film. Hence, microscopicstructural defects at the atomic level are generated at high density atthe interface between the tunnel insulating film and the silicon pillar,and this structural defect forms an interface state or a fixed charge.This decreases the operating speed of the cell transistor constitutingthe memory cell. Furthermore, the threshold voltage of the celltransistor varies after prolonged operation and causes malfunctions.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a method formanufacturing a nonvolatile semiconductor memory device, including:forming a charge storage film, a tunnel insulating film, and a channelfilm made of a silicon crystal in this order on a gate electrode; andforming a silicon oxide layer at an interface between the tunnelinsulating film and the channel film by performing thermal treatment inan oxygen gas atmosphere.

According to an aspect of the invention, there is provided a method formanufacturing a nonvolatile semiconductor memory device, including:forming a charge storage film, a tunnel insulating film, and a channelfilm made of a silicon crystal in this order on a gate electrode; andforming a silicon oxynitride layer at an interface between the tunnelinsulating film and the channel film by performing thermal treatment ina nitrogen monoxide gas atmosphere.

According to an aspect of the invention, there is provided a nonvolatilesemiconductor memory device including: a substrate; a stacked bodyprovided on the substrate, the stacked body including a plurality ofinterlayer insulating films and gate electrodes alternately stackedtherein, and the stacked body including a trench; a charge storage filmprovided on an inner surface of the trench; a tunnel insulating filmprovided on the charge storage film; and a channel film provided on thetunnel insulating film and made of a silicon crystal, one of a siliconoxide layer and a silicon oxynitride layer being formed at an interfacebetween the tunnel insulating film and the channel film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B to 6A and 6B illustrate a method for manufacturing anonvolatile semiconductor memory device according to a first embodimentof the invention, where each figure with the suffix A is a processcross-sectional view, and each figure with the suffix B is a processplan view;

FIGS. 7A and 7B to 8A and 8B are process cross-sectional viewsillustrating a method for manufacturing a nonvolatile semiconductormemory device according to a second embodiment of the invention, whereeach figure with the suffix B is a partially enlarged view of region Rshown in the corresponding figure with the suffix A;

FIGS. 9A and 9B to 14A and 14B illustrate a method for manufacturing anonvolatile semiconductor memory device according to a third embodimentof the invention, where each figure with the suffix A is a processcross-sectional view, and each figure with the suffix B is a processplan view;

FIGS. 15A, 15B, 16A, and 16B illustrate a method for manufacturing anonvolatile semiconductor memory device according to a fourth embodimentof the invention, where each figure with the suffix A is a processcross-sectional view, and each figure with the suffix B is a processplan view;

FIG. 17 is a perspective view illustrating a nonvolatile semiconductormemory device according to the fourth embodiment; and

FIG. 18 is a cross-sectional view illustrating a nonvolatilesemiconductor memory device according to a comparative example of thefourth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will now be described with reference to thedrawings.

At the outset, a first embodiment of the invention is described.

FIGS. 1A and 1B to 6A and 6B illustrate a method for manufacturing anonvolatile semiconductor memory device according to this embodiment,where each figure with the suffix A is a process cross-sectional view,and each figure with the suffix B is a process plan view.

First, as shown in FIGS. 1A and 1B, on a silicon substrate 11,illustratively by the CVD process, silicon oxide films 12 to serve asinterlayer insulating films and impurity-doped silicon films 13 to serveas gate electrodes are alternately deposited to form a multilayer body14. The thickness of the silicon oxide film 12 and the silicon film 13is e.g. 50 nm (nanometers) each. Although FIG. 1A shows only two siliconfilms 13, this embodiment is not limited thereto. The number of siliconfilms 13 is arbitrary.

Next, as shown in FIGS. 2A and 2B, a resist mask (not shown) is formedon the stacked body 14 and used as a mask to perform RIE (reactive ionetching). Thus, the silicon oxide films 12 and the silicon films 13 areselectively removed to form a trench 15 in the stacked body 14. Thetrench 15 is shaped like a groove, and its width is e.g. approximately70 nm. The silicon oxide films 12 serving as interlayer insulating filmsand the silicon films 13 serving as gate electrodes are exposed to theside surface of the trench 15. Furthermore, the silicon substrate 11 isexposed to the bottom surface of the trench 15.

Next, as shown in FIGS. 3A and 3B, by the ALD (atomic layer deposition)process, an alumina film 16 having a thickness of e.g. 10 nm to serve asa charge block film, a silicon nitride film 17 having a thickness ofe.g. 5 nm to serve as a charge storage film, and a silicon oxide film 18having a thickness of e.g. 5 nm to serve as a tunnel insulating film aredeposited in this order on the inner surface of the trench 15. Thecharge block film is a film which does not substantially pass a currenteven if a voltage in the operating voltage range of the nonvolatilesemiconductor memory device according to this embodiment is applied. Thecharge storage film is a film capable of storing charge, such as a filmcontaining electron trap states. The tunnel insulating film is a filmwhich is normally insulative, but passes a tunneling current when aprescribed voltage in the operating voltage range of the nonvolatilesemiconductor memory device is applied.

Next, by the CVD process, an impurity-doped amorphous silicon film isdeposited on the silicon oxide film 18 to a thickness of e.g. 10 nm.Here, the source gas used for CVD is illustratively silane (SiH₄). Theamorphous silicon film does not completely fill in the trench 15, but acavity remains in the trench 15. Subsequently, thermal treatment isperformed at a temperature of e.g. 500° C. or more to crystallize thisamorphous silicon film into a polycrystalline silicon film 19.Alternatively, by adjusting the film formation temperature of CVD,polycrystalline silicon can be deposited on the silicon oxide film 18 toform a polycrystalline silicon film 19.

Next, as shown in FIGS. 4A and 4B, a resist mask (not shown) is formedon the stacked body 14 and used as a mask to perform RIE to selectivelyremove the polycrystalline silicon film 19. Thus, the polycrystallinesilicon film 19 is divided along the extending direction of the trench15 into a plurality of U-shaped channel silicon crystal films 20. Here,the space in the trench 15 devoid of the alumina film 16, the siliconnitride film 17, the silicon oxide film 18, and the channel siliconcrystal film 20 constitutes a cavity 23. The cavity 23 communicates withthe outside of the trench 15, and the channel silicon crystal film 20 isexposed to the inner surface of the cavity 23. Although FIG. 4B showsonly three channel silicon crystal films 20, this embodiment is notlimited thereto. The number of channel silicon crystal films 20 isarbitrary.

Next, as shown in FIGS. 5A and 5B, thermal oxidation is performed in anoxygen gas (O₂) atmosphere. The condition for this thermal oxidation issuch that the temperature is e.g. 700 to 1200° C. and the duration ise.g. 1 minute to 1 hour. Thus, a silicon oxide layer 21 a having athickness of e.g. approximately 0.2 to 2 nm is formed at the interfacebetween the channel film (channel silicon crystal film 20) and thetunnel insulating film (silicon oxide film 18). Here, by this thermaloxidation, a silicon oxide layer 21 b is formed also at the exposedsurface of the channel silicon crystal film 20. The thickness of thesilicon oxide layer 21 b is approximately 1.5 to 3 times the thicknessof the silicon oxide layer 21 a, although it depends on the thicknessand thermal treatment condition of the channel silicon crystal film 20.

Next, as shown in FIGS. 6A and 6B, silicon oxide is deposited or appliedon the entire surface to bury a silicon oxide 22 in the cavity 23. Thesilicon oxide 22 constitutes a device isolation insulating film. Next,CMP (chemical mechanical polishing) is performed to remove the siliconoxide 22 and the silicon oxide layer 21 b from above the upper surfaceof the stacked body 14 to expose the upper surface of the channelsilicon crystal film 20. Thus, a memory array section is formed on thesilicon substrate 11. In the memory array section, a NAND string isconstructed for each U-shaped channel silicon crystal film 20. In eachNAND string, a plurality of MONOS type memory cell transistors arearranged and series connected along the extending direction of thechannel silicon crystal film 20. Subsequently, by conventionaltechniques, select gate transistors, interconnect layers, and peripheralelements (all not shown) are formed to manufacture a nonvolatilesemiconductor memory device having a three-dimensional stackedstructure.

As shown in FIGS. 6A and 6B, in this nonvolatile semiconductor memorydevice, a stacked body 14 with a plurality of interlayer insulatingfilms (silicon oxide films 12) and gate electrodes (silicon films 13)alternately stacked therein is provided on a silicon substrate 11, and atrench 15 is formed in the stacked body 14. A charge block film (aluminafilm 16), a charge storage film (silicon nitride film 17), and a tunnelinsulating film (silicon oxide film 18) are provided in this order onthe inner surface of the trench 15, and a channel silicon crystal film20 made of a silicon crystal is provided on the tunnel insulating film(silicon oxide film 18). Furthermore, a silicon oxide layer 21 a isformed at the interface between the tunnel insulating film (siliconoxide film 18) and the channel film (channel silicon crystal film 20).

Next, the function and effect of this embodiment are described.

In this embodiment, in order to efficiently manufacture a nonvolatilesemiconductor memory device having a three-dimensional stackedstructure, interlayer insulating films (silicon oxide films 12) and gateelectrodes (silicon films 13) are alternately deposited to form astacked body 14, and subsequently a trench 15 is formed. Then, a chargeblock film (alumina film 16), a charge storage film (silicon nitridefilm 17), and a tunnel insulating film (silicon oxide film 18) areformed on the inner surface of the trench 15, and a channel film(channel silicon crystal film 20) is formed thereon. That is, in theprocess shown in FIGS. 3A and 3B, an amorphous silicon film is depositedon the silicon oxide film 18 by the CVD process. At this time, at theinterface between the silicon oxide film 18 and the amorphous siliconfilm, hydroxy groups resulting from the CVD source gas (such as silane)remain, and/or dangling bonds are formed. Thus, microscopic structuraldefects are generated. When the amorphous silicon film is crystallizedand processed into a channel silicon crystal film 20, this structuraldefect remains at the interface between the silicon oxide film 18 andthe channel silicon crystal film 20, and generates an interface state ora fixed charge.

Thus, in this embodiment, in the process shown in FIGS. 5A and 5B,thermal treatment is performed in an oxygen gas atmosphere. When thermaltreatment is performed in the condition where the channel siliconcrystal film 20 made of silicon in the crystalline state is exposed tothe oxygen gas atmosphere, a silicon oxide layer 21 b is generated atthe exposed surface of the channel silicon crystal film 20, and asilicon oxide layer 21 a is formed also at the opposite surface, thatis, at the interface with the silicon oxide film 18. This is presumablybecause oxygen gas is resistant to dissociation in the silicon film,diffusing to the opposite surface of the silicon film, where it isdissociated and causes oxidation reaction. Because the silicon oxidelayer 21 a is formed at the interface between the silicon oxide film 18and the channel silicon crystal film 20, microscopic structural defectsgenerated at the interface between the silicon oxide film 18 and thechannel silicon crystal film 20 are captured in the silicon oxide layer21 a. Thus, interface states and fixed charges are significantlyreduced. As an example, the density of interface states and fixedcharges is 1×10¹² cm⁻² or more when no silicon oxide layer 21 a isformed, but decreases to 1×10¹¹ cm⁻² or less when the silicon oxidelayer 21 a is formed.

Thus, according to this embodiment, the microscopic structural state atthe interface between the silicon oxide film 18 serving as a tunnelinsulating film and the channel silicon crystal film 20 serving as achannel film is significantly improved, and hence the operating speed ofthe cell transistor is increased. Furthermore, the threshold voltage ofthe cell transistor is less likely to vary despite prolonged operation,and the reliability of the memory is improved.

Furthermore, in this embodiment, when the thermal treatment shown inFIGS. 5A and 5B is performed, a cavity 23 communicating with the outsideof the trench 15 is formed in the trench 15, and the channel siliconcrystal film 20 is exposed to the inner surface of the cavity 23. Hence,oxygen gas in the atmosphere is efficiently supplied to the entirechannel silicon crystal film 20 through the cavity 23, and the siliconoxide layer 21 a is formed throughout the interface between the siliconoxide film 18 and the channel silicon crystal film 20.

In the above example, an amorphous silicon film is crystallized into apolycrystalline silicon film 19, which is divided into channel siliconcrystal films 20, and subsequently thermally treated in an oxygen gasatmosphere to form a silicon oxide layer 21 a. However, this embodimentis not limited thereto as long as the silicon film is crystallizedbefore thermal treatment in an oxygen gas atmosphere. For instance, theamorphous silicon film can be divided, then crystallized, andsubsequently thermally treated in an oxygen gas atmosphere.Alternatively, the amorphous silicon film can be crystallized, thenthermally treated in an oxygen gas atmosphere, and subsequently divided.However, preferably, thermal treatment in an oxygen gas atmosphere isperformed after the division process. This is because the efficiency ofsupplying oxygen gas is then higher and facilitates forming a siliconoxide layer 21 a, and physical and electrical damage during the divisionprocess can be recovered.

Next, a second embodiment of the invention is described.

FIGS. 7A and 7B to 8A and 8B are process cross-sectional viewsillustrating a method for manufacturing a nonvolatile semiconductormemory device according to this embodiment, where each figure with thesuffix B is a partially enlarged view of region R shown in thecorresponding figure with the suffix A.

First, by a method similar to that in the above first embodiment, astructure shown in FIGS. 4A and 4B is fabricated. Next, as shown in FIG.7A, thermal oxynitridation is performed in a nitrogen monoxide gas (NO)atmosphere. The condition for this thermal oxynitridation is such thatthe temperature is e.g. 700 to 1200° C. and the duration is e.g. 1minute to 1 hour. Here, the nitrogen monoxide gas atmosphere is notlimited to an atmosphere containing only nitrogen monoxide gas, but canbe any atmosphere containing nitrogen monoxide gas.

By this thermal oxynitridation, a silicon oxynitride layer 24 a having athickness of e.g. approximately 0.2 to 2 nm is formed at the interfacebetween the channel silicon crystal film 20 and the silicon oxide film18. At this time, a silicon oxynitride layer 24 b is formed also at theexposed surface of the channel silicon crystal film 20. The thickness ofthis silicon oxynitride layer 24 b is approximately 1.5 to 3 times thethickness of the silicon oxynitride layer 24 a, although it depends onthe thickness and thermal treatment condition of the channel siliconcrystal film 20. Here, the silicon oxynitride layer is formed not onlyat the exposed surface of the channel silicon crystal film 20, but alsoat the interface with the silicon oxide film 18. This is presumablybecause nitrogen monoxide gas is resistant to dissociation in thesilicon film, diffusing from the exposed surface into the silicon filmand reaching the opposite surface of the silicon film, where it isdissociated and causes oxidation and nitridation reaction.

Furthermore, as shown in FIG. 7B, the silicon oxynitride layers 24 a and24 b have a characteristic composition in the thickness direction. Morespecifically, in each of the silicon oxynitride layers 24 a and 24 b, anitrogen-rich layer 24N having a relatively high nitrogen concentrationis formed near the interface with the channel silicon crystal film 20,and an oxygen-rich layer 24O having a relatively high oxygenconcentration is formed in a region distant from the interface with thechannel silicon crystal film 20. This is presumably because in theprocess of forming a silicon oxynitride layer at the surface of thesilicon film by thermal oxynitridation, the nitrogen atom in the siliconoxynitride layer tends to segregate at the interface between the siliconfilm and the silicon oxynitride layer.

Next, as shown in FIGS. 8A and 8B, like the above first embodiment, asilicon oxide 22 is buried as a device isolation insulating film in thecavity 23. Next, CMP is performed to remove the silicon oxide 22 and thesilicon oxynitride layer 24 b from above the upper surface of thestacked body 14 to expose the upper surface of the channel siliconcrystal film 20. Consequently, a memory array section is formed on thesilicon substrate 11. Subsequently, thermal annealing is performed at atemperature of approximately 400° C. to 1000° C. to densify the siliconoxide 22. This can reduce leakage current in the silicon oxide 22 andreduce the frequency of memory malfunctions. Subsequently, select gatetransistors, interconnect layers, and peripheral elements (all notshown) are formed to manufacture a nonvolatile semiconductor memorydevice having a three-dimensional stacked structure.

As compared with the above first embodiment, the nonvolatilesemiconductor memory device according to this embodiment includes asilicon oxynitride layer 24 a, instead of the silicon oxide layer 21 a(see FIG. 6), at the interface between the tunnel insulating film(silicon oxide film 18) and the channel film (channel silicon crystalfilm 20).

Next, the function and effect of this embodiment are described.

In this embodiment, in the process shown in FIGS. 7A and 7B, thermaloxynitridation is performed in a nitrogen monoxide gas atmosphere. Thus,a silicon oxynitride layer 24 a is formed at the interface between thechannel silicon crystal film 20 and the silicon oxide film 18. Thesilicon oxynitride layer 24 a is a stacked film composed of anitrogen-rich layer 24N formed on the channel silicon crystal film 20side and an oxygen-rich layer 24O formed on the silicon oxide film 18side.

Consequently, microscopic structural defects generated at the interfacebetween the silicon oxide film 18 and the channel silicon crystal film20 are captured in the nitrogen-rich layer 24N, and interface states andfixed charges are significantly reduced. As an example, the density ofinterface states and fixed charges is 1×10¹² cm⁻² or more when nosilicon oxynitride layer 24 a is formed, but decreases to 1×10¹¹ cm⁻² orless when the silicon oxynitride layer 24 a is formed. Thus, theoperating speed of the cell transistor is increased. Furthermore,because the nitrogen-rich layer 24N is provided in contact with thechannel silicon crystal film 20, degradation in the microscopicstructural state of the interface, such as increase of interface states,is prevented even if data write/erase operation is repetitivelyperformed on the memory cell. Thus, the reliability of the celltransistor is dramatically improved.

Furthermore, according to this embodiment, the surface of the channelsilicon crystal film 20 is covered with the silicon oxynitride layers 24a and 24 b during thermal annealing for densifying the silicon oxide 22.This can prevent the channel silicon crystal film 20 from decreasing involume by oxidation. Thus, the frequency of malfunctions of the celltransistor can be significantly reduced, and the reliability of thedevice is improved. Furthermore, the nitrogen-rich layer 24N is providedat both interfaces of the channel silicon crystal film 20. This canprevent aggregation and deformation of the channel silicon crystal film20 during thermal annealing.

In this embodiment, in the process shown in FIGS. 7A and 7B, thermaltreatment in a nitrous oxide gas atmosphere, instead of the nitrogenmonoxide gas atmosphere, can achieve a similar effect. In this case, thethickness of the silicon oxynitride layer 24 b formed at the exposedsurface of the channel silicon crystal film 20 tends to increase. Hence,this is favorable to decreasing the finish thickness of the channelsilicon crystal film 20.

Next, a third embodiment of the invention is described.

FIGS. 9A and 9B to 14A and 14B illustrate a method for manufacturing anonvolatile semiconductor memory device according to this embodiment,where each figure with the suffix A is a process cross-sectional view,and each figure with the suffix B is a process plan view.

This embodiment is different from the above first embodiment in that thetrench is shaped like a cylinder rather than a groove, and that thechannel silicon crystal layer is extended to the silicon substrate.

First, as shown in FIGS. 9A and 9B, by a method similar to that in theabove first embodiment, a stacked body 14 is formed on a siliconsubstrate 11.

Next, as shown in FIGS. 10A and 10B, RIE is performed using a resistmask (not shown) to form a plurality of trenches 25 in the stacked body14. The trench 25 is shaped like a cylinder extending in the stackingdirection of the stacked body 14, and its inner diameter is e.g. 70 nm.As viewed in the stacking direction of the stacked body 14, the trenches25 are arranged in a matrix. Furthermore, the silicon substrate 11 isexposed to the bottom surface of each trench 25. For convenience ofillustration, each figure shows only one trench 25, but in practice,numerous trenches 25 are formed.

Next, as shown in FIGS. 11A and 11B, by the ALD process, an alumina film16, a silicon nitride film 17, and a silicon oxide film 18 are depositedin this order on the inner surface of the trench 25. The thicknesses ofthese films are e.g. 10 nm, 5 nm, and 5 nm, respectively. Aftercompletion of the device, these films function as a charge block film, acharge storage film, and a tunnel insulating film, respectively.

Next, as shown in FIGS. 12A and 12B, a resist mask (not shown) is formedon the stacked body 14 and used as a mask to perform RIE. Thus, thesilicon oxide film 18, the silicon nitride film 17, and the alumina film16 are selectively removed from above the bottom surface of the trench25 to expose the silicon substrate 11. At this time, the siliconsubstrate 11 is dug down slightly in the upper surface.

Next, by the CVD process, an impurity-doped amorphous silicon film isdeposited on the inner surface of the trench 25 to a thickness of e.g.10 nm. Subsequently, thermal treatment is performed at a temperature ofe.g. 500° C. or more to crystallize this amorphous silicon film into achannel silicon crystal film 30. The lower end portion of the channelsilicon crystal film 30 is in contact with the silicon substrate 11. Thechannel silicon crystal film 30 is shaped like a cylindrical tube withthe bottom closed, and its inside constitutes a cavity 33. That is, thecavity 33 communicates with the outside of the trench 25, and thechannel silicon crystal film 30 is exposed to the inner surface of thecavity 33. Alternatively, by adjusting the film formation temperature ofCVD, polycrystalline silicon can be deposited on the inner surface ofthe trench 25 to form a channel silicon crystal film 30.

Next, as shown in FIGS. 13A and 13B, thermal oxidation is performed inan oxygen gas (O₂) atmosphere. The condition for this thermal oxidationis such that the temperature is e.g. 700 to 1200° C. and the duration ise.g. 1 minute to 1 hour. Thus, a silicon oxide layer 31 a is formed atthe interface between the channel silicon crystal film 30 and thesilicon oxide film 18 and the like. The thickness of the silicon oxidelayer 31 a is e.g. approximately 0.2 to 2 nm. Here, by this thermaloxidation, a silicon oxide layer 31 b is formed also at the exposedsurface of the channel silicon crystal film 30. The thickness of thissilicon oxide layer 31 b is approximately 1 to 2 times the thickness ofthe silicon oxide layer 31 a, although it depends on the thickness andthermal treatment condition of the channel silicon crystal film 30. Onthe other hand, the silicon oxide layer is scarcely formed at theinterface between the channel silicon crystal film 30 and the siliconsubstrate 11.

Next, as shown in FIGS. 14A and 14B, a method similar to that in theabove first embodiment is used to bury a silicon oxide 22 in the cavity33. Next, CMP is performed to remove the silicon oxide layer 31 b fromabove the upper surface of the stacked body 14 to expose the uppersurface of the channel silicon crystal film 30. Thus, a memory arraysection is formed on the silicon substrate 11. Subsequently, select gatetransistors, interconnect layers, and peripheral elements (all notshown) are formed to manufacture a nonvolatile semiconductor memorydevice having a three-dimensional stacked structure.

As shown in FIGS. 14A and 14B, in the nonvolatile semiconductor memorydevice manufactured in this embodiment, a stacked body 14 with aplurality of interlayer insulating films (silicon oxide films 12) andgate electrodes (silicon films 13) alternately stacked therein isprovided on a silicon substrate 11, and a cylindrical trench 25 isformed in the stacked body 14. A charge block film (alumina film 16), acharge storage film (silicon nitride film 17), and a tunnel insulatingfilm (silicon oxide film 18) are provided in this order on the sidesurface of the trench 25, and a channel silicon crystal film 30 shapedlike a cylindrical tube with the bottom closed is provided inside thetrench 25 and connected to the silicon substrate 11. The inside of thechannel silicon crystal film 30 constitutes a cavity 33 with the topopen, and a silicon oxide 22 is buried inside the cavity 33.Furthermore, a silicon oxide layer 31 a is formed at the interfacebetween the tunnel insulating film (silicon oxide film 18) and thechannel film (channel silicon crystal film 30), and a silicon oxidelayer 31 b is formed at the interface between the buried insulating film(silicon oxide 22) and the channel film (channel silicon crystal film30).

Next, the function and effect of this embodiment are described.

According to this embodiment, it is possible to manufacture anonvolatile semiconductor memory device having a three-dimensionalstacked structure in which the channel of memory cell transistors isconnected to the silicon substrate 11.

In this embodiment, as compared with the above first embodiment, underthe same thermal oxidation condition, the ratio of the thickness of thesilicon oxide layer 31 b to the thickness of the silicon oxide layer 31a can be decreased. For instance, this ratio is from 1.5 to 3 in theabove first embodiment, but can be from 1 to 2 in this embodiment. Thatis, in the channel silicon crystal film 30 shaped like a cylindricaltube, oxidation reaction is suppressed at the inner side surface, butaccelerated at the outer side surface. This is presumably because thechannel silicon crystal film 30 is concave at the inner side surface andconvex at the outer side surface, and hence the internal stressgenerated with the progress of oxidation reaction is increased in theinner side surface and released in the outer side surface. Thus, byrestricting thickening of the silicon oxide layer 31 b and ensuring thethickness of the channel silicon crystal film 30 after thermaloxidation, a desired channel current can be obtained. Furthermore, thethermal oxidation temperature for forming a silicon oxide layer 31 awith a desired thickness can be decreased, and hence thermal diffusionof elements can be restricted. Consequently, the structure of the memoryarray section can be made finer, and a nonvolatile semiconductor memorydevice with fewer memory malfunctions can be manufactured.

The function and effect of this embodiment other than the foregoing arethe same as those of the above first embodiment. More specifically, alsoin this embodiment, a silicon oxide layer 31 a is formed at theinterface between the tunnel insulating film (silicon oxide film 18) andthe channel film (channel silicon crystal film 30). Thus, interfacestates and fixed charges at this interface are reduced, and thisembodiment can manufacture a nonvolatile semiconductor memory device inwhich the cell transistor is fast and has little threshold voltagevariation. Also in this embodiment, like the above first embodiment, bythermal treatment in an oxygen gas atmosphere, the density of interfacestates and fixed charges can be decreased, for instance, from 1×10¹²cm⁻² or more to 1×10¹¹ cm⁻² or less.

In the example described in this embodiment, like the above firstembodiment, thermal treatment is performed in an oxygen gas atmosphereto oxidize the surface of the channel silicon crystal film 30.Alternatively, like the above second embodiment, thermal treatment canbe performed in a nitrogen monoxide gas atmosphere or a nitrous oxidegas atmosphere to oxynitridize the surface of the channel siliconcrystal film 30. Thus, the function and effect similar to those of theabove second embodiment can be achieved. Also in this case, in thechannel silicon crystal film 30, oxynitridation reaction is suppressedat the inner side surface, but accelerated at the outer side surface.Hence, an effect similar to the foregoing can be achieved.

Furthermore, in this embodiment, a cylindrical tubular channel siliconcrystal film 30 connected to the silicon substrate 11 is illustrativelyformed. However, this embodiment is not limited thereto. For instance, acylindrical tubular channel silicon crystal film can be bent into aU-shape and electrically insulated from the silicon substrate.

Next, a fourth embodiment of the invention is described.

FIGS. 15A, 15B, 16A, and 16B illustrate a method for manufacturing anonvolatile semiconductor memory device according to this embodiment,where each figure with the suffix A is a process cross-sectional view,and each figure with the suffix B is a process plan view.

FIG. 17 is a perspective view illustrating a nonvolatile semiconductormemory device according to this embodiment.

In FIG. 17, for convenience of illustration, the silicon substrate 11and the silicon oxide film 12 are not shown.

This embodiment is different from the above third embodiment in thatpolycrystalline silicon, rather than the silicon oxide 22 (see FIG. 14),is buried inside the channel silicon crystal film 30 shaped like acylindrical tube.

First, by a method similar to that in the above third embodiment, astructure shown in FIGS. 13A and 13B is fabricated. Next, as shown inFIGS. 15A and 15B, wet etching is performed using a dilute hydrofluoricacid solution to remove the silicon oxide layer 31 b formed at theexposed surface of the channel silicon crystal film 30. Here, in thecase where a silicon oxynitride layer is formed at the exposed surfaceof the channel silicon crystal film 30 by thermal oxynitridation in anatmosphere containing nitrogen monoxide gas or nitrous oxide gas, thissilicon oxynitride layer is removed by wet etching using a phosphoricacid solution. Thus, the channel silicon crystal film 30 is exposed tothe inner surface of the cavity 33.

Next, as shown in FIGS. 16A and 16B, impurity-doped amorphous silicon isdeposited on the entire surface and buried inside the cavity 33.Subsequently, CMP is performed to remove the amorphous silicon depositedon the stacked body 14 to expose the upper surface of the channelsilicon crystal film 30. Next, thermal annealing is performed at atemperature of e.g. 500° C. or more to crystallize the amorphous siliconinto polycrystalline silicon 32. Subsequently, select gate transistors,interconnect layers, and peripheral elements (all not shown) are formedto manufacture a nonvolatile semiconductor memory device having athree-dimensional stacked structure.

As shown in FIGS. 16A, 16B, and 17, in the nonvolatile semiconductormemory device according to this embodiment, the cylindrical tubularchannel silicon crystal film 30 formed on the side surface of the trench25 and the polycrystalline silicon 32 buried therein constitute avertically extending silicon pillar connected to the silicon substrate11. This silicon pillar functions as a channel of cell transistors.

Next, the function and effect of this embodiment are described.

In this embodiment, as compared with the above third embodiment, thesilicon pillar serving as a channel has a larger cross-sectional area,hence allowing a large cell current to flow therein. Consequently, theoperating speed of the cell transistor is significantly increased. Thefunction and effect of this embodiment other than the foregoing are thesame as those of the above third embodiment.

In this embodiment, a cylindrical tubular channel silicon crystal film30 connected to the silicon substrate 11 is illustratively formed.However, this embodiment is not limited thereto. For instance, acylindrical tubular channel silicon crystal film can be bent into aU-shape and electrically insulated from the silicon substrate.

Next, a comparative example of the fourth embodiment is described.

FIG. 18 is a cross-sectional view illustrating a nonvolatilesemiconductor memory device according to this comparative example.

As shown in FIG. 18, also in this comparative example, like the abovefourth embodiment, a trench 25 is formed in the stacked body 14, and analumina film 16, a silicon nitride film 17, and a silicon oxide film 18are formed in this order on the inner surface of the trench 25. Next, anamorphous silicon film is formed by CVD on the silicon oxide film 18 andcrystallized into a cylindrical channel silicon crystal film 40.However, in this comparative example, the formation of the amorphoussilicon film is not followed by thermal treatment in an oxygen gasatmosphere or in an atmosphere containing nitrogen monoxide gas ornitrous oxide gas. Hence, no silicon oxide layer or silicon oxynitridelayer is formed at the interface between the silicon oxide film 18 andthe channel silicon crystal film 40.

In this comparative example, microscopic structural defects generatedduring deposition of the amorphous silicon film on the silicon oxidefilm 18 remain to the end and generate interface states and fixedcharges at the interface between the silicon oxide film 18 and thechannel silicon crystal film 40. This decreases the operating speed ofthe cell transistor. Furthermore, threshold voltage variation becomesnoticeable after prolonged operation and causes malfunctions in thememory cell. In contrast, according to the above first to fourthembodiment, thermal treatment is performed in an oxygen gas atmosphereor in an atmosphere containing nitrogen monoxide gas or nitrous oxidegas to form a silicon oxide layer or silicon oxynitride layer at theinterface between the silicon oxide film constituting the tunnelinsulating film and the channel silicon crystal film constituting thechannel film. Hence, interface states and fixed charges at thisinterface can be reduced.

The invention has been described with reference to the embodiments.However, the invention is not limited to these embodiments. Forinstance, the invention is also applicable to the case where the memorycell is an MNOS (metal-nitride-oxide-silicon) type memory cell in whichthe charge block film is omitted. Furthermore, it is necessary for thechannel film to contain silicon as principal component, but theinvention is also applicable to the case where the channel film containsdopant elements, such as boron or phosphorus, and contains germanium,carbon, nitrogen or oxygen. Furthermore, those skilled in the art cansuitably modify the above embodiments by addition, deletion, or designchange of components, or by addition, omission, or condition change ofprocesses, and such modifications are also encompassed within the scopeof the invention as long as they fall within the spirit of theinvention.

The invention claimed is:
 1. A nonvolatile semiconductor memory devicecomprising: a substrate; a stacked body provided on the substrate, thestacked body including a plurality of interlayer insulating films andgate electrodes alternately stacked therein, and the stacked bodyincluding a trench; a charge storage film provided on an inner surfaceof the trench; a tunnel insulating film provided on the charge storagefilm; and a channel film provided on the tunnel insulating film and madeof a silicon crystal, the tunnel insulating film including: a siliconoxide layer formed on the charge storage film side; a silicon oxynitridefilm formed on the channel film side, the silicon oxynitride filmincluding, a nitrogen-rich layer formed on the channel film side; and anoxygen-rich layer formed on the charge storage film side, thenitrogen-rich layer having a higher nitrogen concentration than thenitrogen concentration of the oxygen-rich layer.
 2. The device accordingto claim 1, further comprising a charge block film between the gateelectrodes and the charge storage film.
 3. The device according to claim2, wherein the charge block film contains alumina.
 4. The deviceaccording to claim 1, further comprising an insulating film on thechannel film.
 5. The device according to claim 4, wherein the insulatingfilm contains silicon oxide.
 6. The device according to claim 4, whereinthe nitrogen-rich layer and the oxygen-rich layer are provided alsobetween the channel film and the insulating film.
 7. The deviceaccording to claim 1, wherein the charge storage film contains siliconnitride.
 8. The device according to claim 1, wherein a shape of thechannel film is U-shape as seen from a direction in which the trenchextends.
 9. The device according to claim 1, further comprising: anotherchannel film; and an insulating film disposed between the channel filmand the another channel film in the trench.
 10. The device according toclaim 1, wherein a shape of the trench is a cylinder.